The present invention is directed to aqueous ink jet ink compositions for use with commercial offset media and offset ink.
In recent years, computer printer technology has evolved to a point where very high resolution images can be transferred to various media, including papers of different types. One particular type of printing involves the placement of small drops of a fluid ink onto a surface in response to a digital signal. Typically, the fluid ink is placed or jetted onto the surface without physical contact between the printing device and the surface. The specific method for which the ink is deposited onto the printing surface varies from system to system. However, two major methods include continuous ink deposit and drop-on-demand ink deposit.
With regard to continuous printing systems, inks used are typically based on solvents including methyl ethyl ketone and/or ethanol. Essentially, continuous printing systems function as a stream of ink droplets are ejected and directed by a printer nozzle. The ink droplets are directed additionally with the assistance of an electrostatic charging device in close proximity to the nozzle. If the ink is not used on the desired printing surface, the ink is recycled for later use. With regard to drop-on-demand printing systems, the ink jet inks are typically based upon water and glycols. Essentially, with these systems, ink droplets are propelled from a nozzle by heat or by a pressure wave. Additionally, all of the ink droplets are used to form the printed image and are ejected when needed.
In general, ink jet inks are either dye-based or pigment-based. Dye-based inks typically use a liquid colorant that is usually water-based to turn the media a specific color. Because of their makeup, dye-based inks are usually not waterproof and tend to be more affected by UV light. This results in the color fading or changing over time. For optimum performance, this type of ink has often required that the proper media or substrate be selected in accordance with the application. In many circumstances, if the media is too dense or hydrophobic, the ink has difficulty penetrating and beads on the surface. Conversely, if the media is too absorbent, the dot gain is too high creating a blurred image.
Pigmented inks typically use a solid colorant to achieve color. In many cases, the line quality and accuracy of plots produced by pigment-based inks are usually superior to that of dye-based inks. With pigmented inks, solid particles adhere to the surface of the substrate. Once the water in the solution has evaporated, the particles will generally not go back into solution, and are therefore more waterproof. In addition, pigmented inks are much more UV resistant than dye-based inks, meaning that it takes much longer for noticeable fading to occur.
Though pigmented inks, in some areas, exhibit superior characteristics, dyes tend to run cleaner, provide better yield, offer better particle size, and are easier to filter. Thus, dye based inks have been more often used for common applications. Additionally, dye-based inks have tended to be more chromatic and provide more highly saturated colors.
There are several reasons that ink jet printing has become a popular way of recording images on surfaces, particularly paper. Some of these reasons include low printer noise, capability of high speed recording, and multi-color recording. Additionally, these advantages can be provided at a relatively low price. However, though there has been great improvement in ink jet printing, accompanying this improvement are increased printing demands, e.g., higher speed, higher resolution, full color image formation, etc. As such, there are several features to consider when evaluating a printer ink in conjunction with a printing surface or substrate. Such features include edge acuity and optical density of the image on the surface, dry time of the ink on the substrate, adhesion to the substrate, lack of deviation of ink droplets, presence of all dots, resistance of the dried ink to water and other solvents, long-term storage stability, and long-term reliability without corrosion or nozzle clogging. Though the above list of features provides a worthy goal to achieve, there are difficulties associated with satisfying all of the above features. Often, the inclusion of an ink component meant to satisfy one of the above characteristics can prevent another characteristic from being met. Thus, most commercial inks for use in ink jet printers represent a compromise in an attempt to achieve at least an adequate response in meeting some or all of the above listed requirements.
Papers used for ink jet printing have typically included high-quality or wood-free papers designed to have high ink absorptivity or papers having a coated porous surface. These papers are functionally good for ink jet printing because the inks may be absorbed readily and dry quickly. However, such papers often do not allow for a crisp or sharp image.
Conversely, with commercial offset paper, a nonporous smooth surface may provide a good printing surface for a crisp image. However, commercial offset coated papers are significantly different than office plain papers or photo/glossy papers specifically designed for ink jet media. Typically, with commercial offset papers, the smooth non-porous surface is comprised of a coating which requires more time for aqueous fluids to penetrate than standard paper. This is because diffusion-type adsorption must generally occur with offset papers as compared with capillary-type absorption which typically occurs with respect to standard office paper and some ink jet specialty papers. Additionally, offset coatings contain polymers that are more hydrophobic, e.g., styrene-butadiene based, than paper coatings specifically designed for ink jet ink, e.g., water-soluble polymers such as polyvinyl alcohol. Thus, because offset coatings are typically hydrophobic, have poor penetration, and are smooth and non-porous, these coatings tend to interact poorly with water-based inks. In addition, classic ink jet solvents such as glycols and diols tend to perform poorly on these coatings, showing long dry times and poor spreading characteristics.
The apparent incompatibility between offset media/ink and water based ink jet inks stems from the fact that offset media such as commercial offset paper was developed primarily for use with oil-based inks. For example, coated offset media often includes a hydrophobic component such as latex binder and/or various hydrophobic polymers. To illustrate, such polymers used in offset media can include polymers, copolymers, and/or terpolymers selected from the group consisting of polystyrene, polyolefins (polypropylene, polyethylene, polybutadiene), polyesters (PET), polyacrylate, polymethacrylate, and poly(maleic anhydride).
Because commercial offset paper provides a smooth surface for printing and would provide a convenient and inexpensive alternative to specialty papers, it would be useful provide aqueous based ink jet inks which can be used with commercial offset media, including papers and offset inks. Such formulations would be particularly useful if they exhibited a reduction in ink dry time after printing. Additionally, these ink jet inks would also be desirable if they exhibited printing properties on offset media including excellent text and area fill, minimal coalescence in half tone images, excellent optical density (OD) and edge acuity, good water fastness, good smudge and rub resistance, and good lightfastness.
The present invention is drawn to aqueous based ink compositions for ink jet printing on offset media and offset ink. One embodiment comprises effective amounts of a pigment combined with an effective amount of a dye as the ink colorants to achieve a higher optical density and neutral shade across the offset media while exhibiting good waterfastness and lightfastness. This addition of dye to the pigmented ink increases the optical density on the substrate and allows for the use of increased amounts of surfactants. Additionally, the combination of colorants produces an ink with better color neutrality and degree of coloration versus uses of only one type of colorant material. Optionally, additional dispersants, solvents, and binders can be added. Another embodiment comprises an effective amount of a combined pigment/dye colorant and an effective of surfactant to decrease dry time. The pigment is present in an amount of from about 0.5% to about 5%, preferably from about 2.5% to about 4%, by wt of ink composition. The dye is present in an amount of from about 0.1% to 5%, preferably from about 0.5% to about 2%, by weight of ink composition. The surfactant, if present, ranges in an amount of from about 0.1% to about 1.5%, preferably from about 0.4% to about 1.5%, by wt of ink composition.